From my perspective, all the streaming services should be working together to create a healthy marketplace for independent music, and providing every opportunity they can for musicians to avoid signing a big-four contract. If Warner and Sony, and UMG get their act together and create a joint venture streaming service that's the exclusive host of the content they own, Spotify, Google Music, and Apple Music are screwed.
Unfortunately for music, the exclusivity has started to show up in some cases. Artists that own the Tidal service don't allow any of their music on competing services, and Apple has been signing contracts with musicians to have new content show up on their service several months in advance before it does on other services (like Jay Z and Kanye).
From Netflix subscriber numbers, it's also not clear that the lost content has dented their subscriber retention at all. The talk among my friends centers much more around new Netflix originals (or even Amazon/Hulu originals) than around back catalog stuff on Amazon or Hulu. I don't understand why -- I'm utterly uncompelled by 95%+ of what they're putting out, since I don't have the motivation to deeply investigate new Netflix shows/movies every week, and however they're advertising, it doesn't really reach me -- but it's what I see happening, much to my annoyance.
If they sign on 100 top artistes, it will devalue the portfolio of music held by the top 3 - for reasons you explained. Thus, the top 3 are incentivized to collaborate with Spotify.
Making their own content for video has the advantage that most people are OK with opening a new browser tab or an app to watch a specific movie or show.
With music, however, you don't want to run multiple conflicting music players, especially when they don't sync up playlists and seamlessly work together.
I wonder how that will pan out. It's a significant difference.